Fluid catalytic cracking processes for producing gasoline from gas-oil feedstocks are conventionally carried out as pressures within the range of 8 to 20 p.s.i.g. at 850.degree. to 950.degree.F. Catalyst to oil ratios are generally within the range of 8 to 18 (weight basis). The catalysts for such cracking processes are frequently supplied in the form of very small essentially spherical particles predominantly within the range of 100/325 mesh (Tyler). These particles are usually called "microspheres." The microspheres must possess certain properties among which is hardness, especially resistance to attrition. The particles must also possess adequate activity and selectivity as well as thermal and steam stability.
Highly active and selective cracking catalysts have been prepared by incorporating finely divided crystals of certain crystalline zeolitic aluminosilicates of the molecular sieve type with suitable matrix material such as clay or silica-alumina gels. Ion-exchange synthetic faujasite (zeolite X or zeolite Y) is a suitable crystalline zeolite.
Faujasite may be synthesized from dilute high purity sources of Na.sub.2 O, Al.sub.2 O.sub.3 and SiO.sub.2. It may also be synthesized by reacting sodium hydroxide solution with calcined kaolin clay. However, in order to obtain faujasite, rather than other zeolitic molecular sieves, the clay must be calcined at relatively high temperature and undergo the characteristic kaolin exotherm. The incorporation of small amounts of kaolin clay calcined at lower temperature ("metakaolin") may aid in the crystallization of the faujasite but, when used as the sole source of silica and alumina, does not result in the crystallization of faujasite.
Microspherical zeolitic cracking catalyst particles have been obtained by forming a slurry of previously formed powdered zeolite crystal with dilute silica hydrosol or alumina-silica hydrosol and spray drying the slurry. Catalyst particles obtained by such processing are expensive since expensive high purity materials are required to prepare the zeolite and a separate binding step is required. The attrition resistance of the products obtained by binding sieve crystals generally leaves something to be desired.
Attempts have also been made to form zeolitic molecular sieve compositions from preformed bodies of essentially the same size and shape as the finished catalyst particles. This has been successfully accomplished by extruding a mixture of caustic solution and clay material, part of which is anhydrous and calcined, and part of which is hydrated. The extruded pellets, generally in the shape of cylinders, are subjected to hydrothermal treatment without dehydration. Faujasite crystallizes in situ in the presence of hydrated kaolin clay as a result of the reaction between the caustic and the calcined clay in the pellets. A feature of the process is that reaction and crystallization are carried out in the absence of an external aqueous phase in contact with the pellets. Thus, there is no extraction of constituents of the pellets. The catalyst product obtained by ion-exchanging and activating the crystallized product has a SiO.sub.2 /Al.sub.2 O.sub.3 mole ratio of about 2/1 -- i.e., especially the same ratio that is present in kaolin clay.
Great difficulty has been experienced in preparing crystalline zeolitic molecular sieve composite catalysts in the form of microspheres by the in situ process above described. Problems have been encountered in obtaining discrete microspheres and in crystallizing faujasite from preformed particles containing caustic solution and a mixture of calcined kaolin clay and hydrated kaolin clay. Especial difficulty has been encountered in obtaining small spherical particles of desired resistance to attrition. One reason for the difficulties is that when faujasite is crystallized in situ in the presence of hydrated kaolin in microspherical preforms the small spheres tend to agglomerate and stick to each other during reaction and crystallization. On the other hand, impregnation of caustic solution into preformed microspheres composed of the mixture of hydrated and calcined clay tends to result in the mechanical breakdown of the preforms unless great care is exercised. When caustic is included in the feed to the spray dryer so that it is present in the preformed microspheres, part of the caustic tends to react with combustion gases in the spray dryer and difficulty may be experienced crystallizing the zeolite.